A turbine is a machine that comprises a rotary element that moves under pressure of fluid flow to generate useful work which is usually the generation of electricity. Fluid may be gas or liquid or other non-solid phases. One example of a turbine is a wind turbine. When a wind turbine is mentioned in the present disclosure, the fluid concerned is the air, and wind is the flow of air. Wind turbines are commonly horizontal axis or vertical axis, although other types of design have been proposed. The present disclosure can apply to any type of turbine.
A horizontal axis wind turbine is well known to someone skilled in the art. The method of Doppler anemometry is also well known to an individual skilled in the art. It will be appreciated that LIDAR (Light Detection and Ranging) Doppler anemometry systems may employ range gated, pulsed laser beams or alternatively focused continuous wave (CW) laser beams, typically employing substantially co-located emitter and receiver optics, in order to measure velocity components of radial extent along the laser beam direction.
It will be appreciated that emitter and receiver optics do not necessarily need to be substantially co-located and that bi-static configurations may employ emitter and receiver optics substantially displaced from each other.
A person skilled in the art will be familiar with the design of horizontal axis wind turbine including a nacelle housing mounted atop a vertical tower and containing a near-horizontal axis rotating shaft driven by a rotor system attached to the nacelle at a hub from which protrude a plurality of aerodynamically designed blades. A person skilled in the art will also be familiar with pitch control systems which may rotate or pitch the blades through different angles about their longitudinal axes radiating laterally from the near-horizontal drive train axis. It is known that such systems may include a low speed shaft, gearbox and one or more high speed shafts driving generators, or alternatively that the system may be of direct drive type without need for gearbox and high speed shaft. Alternatively, it is also known that hydraulic drive train designs may be implemented within a wind turbine. A person skilled in the art will be familiar with designs where the nacelle housing may be driven to rotate or yaw around a vertical axis such that the rotor axis aligns substantially with the wind direction and the turbine may be driven to face into or away from the wind. It will be appreciated that all the aforementioned drive train types make use of control systems dependent upon wind speed and wind direction measurements and that the usual measurement method currently employed makes use of wind vane or anemometry instruments mounted on the wind turbine nacelle.
Present nacelle mounted Doppler anemometry or LIDAR systems for horizontal axis wind turbines sample only the radial line of sight wind velocity at a given point. However, the three dimensional wind velocity field variation is significant to the operation of a wind turbine. Present techniques collect wind velocity component information by use of a plurality of divergent beams but this results in collection of different velocity components at widely separated points in the incident wind field. The present techniques may make the assumption that the wind velocity field is substantially parallel and uniform. Since it is well known that substantial variation in wind field is possible across a wind turbine rotor swept area this assumption of uniformity or averaging of the wind field leads to a degradation of information resolution and precludes the possibility of detailed measurement of a variable wind velocity vector field.
Measurement of turbulence intensity at a point in space within a given time-averaging period, defined as the standard deviation of wind speed samples divided by the average of the wind speed samples within the time-averaging period, will also be subject to increased measurement error when velocity components are sampled not at the intended measurement point but substantially displaced from the intended measurement point. The standard deviation is then calculated over a data population extending in space as well as over the time-averaging period. It is noted that classical spinning cup or ultrasonic anemometry involves the collection of data substantially at a single point in space.
Various measurement systems have been proposed, in US2012/0051907 (ROGERS); GB2477529 (VESTAS); EP1460266 (MITSUBISHI); WO2011/096928 (CATCH THE WIND); US2012/0274937 (HAYS); and US2013/0162974 (DAKIN); but these all suffer from one or more disadvantages.
Therefore a new fluid velocity measurement system would be beneficial.